The present invention relates to buried heterostructure semiconductor lasers in which a radiative recombination active layer is buried in semiconductor materials which are larger in bandgap but smaller in refractive index than the active layer. More particularly, the present invention is concerned with an improved semiconductor laser having a desirable efficiency, CW (continuous wave) operating temperature and output characteristic.
Buried heterostructure semiconductor laser diodes (BH-LD) are particularly attractive as light sources for long-distance, largecapacity optical fiber communication systems due to their low lasing threshold current, stable fundamental lateral mode operation and their CW operability at high temperature.
The inventors of the present application achieved an BH-LD excellent in temperature characteristics and reproducibility, in which a current blocking layer is formed on both sides of a mesa stripe which includes a radiative recombination active layer, as disclosed in Japanese Patent Application No. 55-48665/1980, laid open on Nov. 13, 1981 as Japanese Patent Laid Open Publication No. 56-146288 (corresponding to U.S. Pat. No. 4,425,650). A BH-LD using an InP substrate and a InGaAsP system material accomplished a lasing threshold current of 20 mA, a differential quantum efficiency of 60% and a maximum CW operating temperature on the order of 100.degree. C.
However, this type of BH-LD is not satisfactory with regard to the reproducibility of its characteristics. Taking the injection current to light output characteristic, for example, experiments have shown that the BH-LD has a sharp tendency to output saturate as the light output power per facet approaches 15-20 mW and becomes hardly operable for light output power equal to or larger than 30 mW per facet. It was found that the saturation of light output power results from a deficiency in the blocking layer structure.